Method for purifying materials



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Jan. 30, 1962 F. D. Rosl ErAL METHOD FOR PURIFYING MATERIALS Filed Dec. 19, 1958 2 Sheets-Sheet 1 INV EN TOR5 FRED D. Rus: 46' LEDNHRD RWI-1 :BERE

MEA/1 Jan. 30, 1962 F. D. ROSI ETAL METHOD FOR PURIFYING MATERIALS Filed Deo. 19, 1958 2 Sheets-Sheet 2 lily-M ff; looooooao l 456 43 INVENTUM FRED D. RDSI 5 LEUNHRD KWI-ISBERE paf/Vr United States Patent Brunswick, NJ., assignors to Radio Corporation of America, a corporation of Delaware Filed Dec. 19, 1958, Ser. No. 781,720 6 Claims. (Cl. 2li-294) This invention relates to the purification of materials by refining methods and more particularly to an improved method for refining volatile solid materials.

Volatile materials are generally refined by means of direct distillation. In such a process, the solid material is heated in excess of its temperature of vaporization in one portion of a closed system. The vaporized material is then cooled and resolidified in another portion of the system. When such a material is distilled, the first portion of the distillate will include impurities having vapor pressures greater than that of the material itself. At the halfway mark in the distillation process, the distillate then being produced will contain significantly less of the impurities having high vapor pressures but will have begun to include impurities having lower vapor pressures. Finally, the last portion of the material being distilled will be substantially free from high vapor pressure impurities but will include `a substantial quantity of low vapor pressure impurities.

Refinement of the material can be improved by repeated fractional distillation. In this procedure, the distillation is generally stopped after the first portion of the distillate has been produced and that portion is then discarded. Distillation is then continued until a second portion of distillate is produced, the remaining undistilled material being discarded. The second portion is again distilled in a like manner to produce a new portion of distillate having a lower impurity content. This procedure is continued until a distillate of the required purity is obtained.

In the refinement of volatile materials having the degree of purity essential when such materials are to be incorporated in semiconductor devices, the above methods become tedious and expensive. Such methods require elaborate distillation apparatus. Materials must be distilled many times to obtain the required purity. Also, when such material is repeatedly handled between distilling operations the risk of external contamination during such handling is increased.

Accordingly, it is a general object of this invention to provide improved methods for refining volatile materials.

It is another object of this invention to provide improved refining methods which employ simplified apparatus.

Yet another object is to provide improved refining methods wherein the risk of external contamination of the material being refined is minimized.

These and other objects and advantages are obtained in accordance with this invention which comprises methods of vaporzone refining of volatile materials. A charge of the material to be refined is positioned in an elongated vessel in a manner such that one end of the vessel is filled with the charge and an empty space is left at the other end. The vessel is then evacuated and the temperature of the end of the charge adjacent to the empty space is raised to at least the temperature of vaporization of the material to create a vapor zone of the material. The vapor zone is then caused to travel through the length of the charge, first filling the empty space with resolidified material and upon completion of travel providing a resolidied charge which has, in effect, been moved one vapor zone length within the vessel.

In a preferred embodiment of this invention, the charge ice of material is inserted into an elongated tubular vessel which is closed at one end. A close fitting plunger is inserted into the other end of the vessel and positioned a distance from the charge substantially equal to the length of the vapor zone to be passed through the length of the charge. This method may be repeated as many times as desired requiring only the simplest of manipulations and equipment.

In another embodiment, an elongated vessel is em ployed which comprises a plurality of substantially identical elongated tubular sections. One section is substantially filled with a charge of the material and a second `section is mated, end for end, to the first section. Both sections are then evacuated and a vapor zone caused to travel through the length of the charge. In this ernbodiment, the vapor zone is substantially equal in length to a single tubular section. When the vapor zone has been passed through the length of the charge, the resolidified charge has, in effect, been transferred from the first to the second tubular section. This method can be con vieniently repeated after exchanging the two sections end for end and again passing a vapor zone through the solid charge.

Other objects and advantages will become apparent from the following detailed description when read in conjunction with the drawings wherein:

FIGURES la, 1b, and lc represent, schematically and in section, a vapor zone travelling through the length of a charge of volatile material in accordance with a preferred method of this invention, together with an apparatus useful in practicing the method.

FIGURES 2a, 2b, and 2c represent, schematically and in section, a second method of zone refining a volatile material in accordance with this invention, together with a second apparatus useful in practicing this second method.

Similar reference characters apply to similar elements throughout the drawings.

An apparatus suitable for practicing the preferred method of this invention, illustrated in FIGURE la, comprises an elongated tube 11 of heavy-walled Pyrex. The tube 11 preferably has a ratio of length to internal diameter as large as is conveniently possible. For example, convenient dimensions include a tube length of 30" and an internal diameter of 0.5000;L0.0002. The tube is equipped with two graphite plungers 13 and 15 which closely fit the Walls of the tube. Rods 17 and 19 connect with plungers i3 and l5 respectively and communicate to the outside of the tube through either end thereof. O rings 21 are positioned near each end of the tube to provide a hermetic seal and yet permit movement of the rods 17 and 19 and of the plungers 13 and l5 inside the tube. I One or more ducts 23 are provided communi eating to the interior of the tube inside the O rings 21. By means of these ducts the tube may be evacuated or supplied with an inert gas. A heater 25 closely surrounds the tube and is movable along the length ot the tube. Such a heater is easily constructed having a length of about l/z and using nichrome wires for the heating elements.

In practicing this method of the invention, one plunger, such as 13, and its associated O ring 2l are removed from the tube. A charge 26 of solid material such as, for example, arsenic is inserted into the tube. The plunger 13 and its associated O ring 21 are again inserted in the tube and the plunger i3 positioned so as to leave a space 27 between it and the charge of arsenic. Such a space may conveniently have a length of about 11A". The tube is then evacuated through one of the ducts 23 and the heater 25 positioned over the space 27. The heat is then energized and brought to a temperature of about 800 C. This will provide a temperature Within the space 27 of at 3 least 450 C. The heater 25 is then moved toward the right, preferably at a speed at about 1 1/2" per hour.

As shown in FIGURE 1b, during movement of the heater `25 along the length of the charge 26, solid arsenic is vaporized to form a vapor zone 31 about 1%. in length. As the heater continues its travel along the tube, solid arsenic deposits out, initially on the inner face of the graphite plunger 13, and then successively in a solid mass 26 behind the vapor zone 31.

As shown in FIGURE lc when a complete pass of the heater 25 along the length of the charge 26 has been cornpleted, the arsenic has been resolidified into a solid mass 2% which has, in effect, been moved to the left within the tube a distance of one zone length leaving a void 35 adjacent the right plunger 15. To prepare for making a second pass over the solid charge the graphite plunger is moved inward until it contacts the solid charge 26 thereby closing up the void 35 which was created upon completion of the first pass.

The plunger 13 is again positioned within the tube to provide the configuration illustrated in FIGURE 1. The plunger 13 is withdrawn about one zone length so as to provide the space 27 between it and the resolidified charge of solid arsenic.

In the purification of some materials, such as arsenic, vapor may deposit on the walls of tube 11 between those walls and the plunger 13 causing the plunger to become frozen. In order to facilitate withdrawal of the plunger the heater can be positioned over the plunger for about five minutes after which the plunger is free and can be easily moved. The process is then repeated as many times as desired to obtain material of the required purity.

During each pass of the vapor zone 31 through the solid charge 26, impurities having a Vapor pressure greater than that of the material being refined will be moved in the direction of Zone travel and will ultimately be deposited at the end of the charge adjacent the plunger 15. During each pass of the vapor zone 31 through the solid charge 26, those impurities having a vapor pressure less than that of the material being rened will deposit behind the zone and will therefore be moved one zone length toward the plunger 13 for each pass made through the charge. Thus, with sufiicient passes low vapor pressure materials will be deposited at the end of the charge adjacent the plunger 13 and those having high vapor pres-sures will be deposited adjacent plunger 15. Finally, the purified charge of material is removed from the tube and both ends thereof, containing the impurities, are cut ofi. Where difficulty is encountered in removing the charge from the tube, this can easily be accomplished, with materials such as arsenic, by dipping the tube into liquid nitrogen thereby causing the arsenic charge to contract whereupon it is readily removable.

`In the purification of arsenic it has been found convenient to provide an initial charge 26, in lump form, about 12 in length. The vapor zone 31 was passed through the charge 26 at a rate of about 11/2" per hour. After two passes of the vapor zone 31 through the charge 26 its length was reduced to about 7" and the arsenic was in the form of a solid, substantially homogeneous, rod completely filling the cross section of the tube 11. After each pass of the vapor zone 31, the right plunger 15 was moved inward to contact the resolidified charge of arsenic and the left plunger 13 was withdrawn a distance of about 1%". Only six passes of the vapor Zone 31 through the charge 26 were required to complete the purification process. Finally, the purified charge of arsenic was removed from the tube and about 11/4" cut from each end providing a solid rod about 41/2 long of substantially pure arsenic. In the purification of arsenic, it has been found that, when vapor Zone speeds in excess of ll/z per hour are employed, the resolidified arsenic has a tendency to take the form of a hollow cylinder. When it is desired to avoid such a result, zone speed and withdrawal distance ofthe left plunger 13 should be carefully controlled, particularly during the least few passes of the vapor zone 31 through the charge 26.

In FIGURE 2a, there is illustrated an apparatus suitable for use in another method of this invention. This apparatus comprises a tubular section 41 which is substantially filled with a charge of arsenic 42. A substantially identical tubular section 43 is joined to the filled section 41. The joining ends of the two sections are designed to provide a vacuum type seal 45 such as a ground glass tapered joint. A nose cap 46 is inserted over the other end of the filled section 41 and again a ground glass vacuum type seal 47 is provided. The empty section 43 is provided at its other end with a plug 49 again employing a ground glass vacuum type seal 51. The plug 49 is hollow and is provided with apertures 53 communicating to the interior of the empty section 43. The plug 49 can thereby be connected to a vacuum pump to provide for the evacuation of the filled section 41 as well as the empty section 43. A heater unit S5 closely surrounds the tubular sections 41 and 43 and has a length substantially equal to thatV of the charge within the filled section ,41.

As illustrated in FIGURE 2a, the heater element y55 is first positioned over the empty section 43,. It is then caused to travel to the left to successively vaporize increasing proportions of the solid mass 42. As illustrated in FIGURE 2b, during travel of the zone over the solid charge 42, arsenic is vaporized and maintained in a vapor state within the length covered by the heater element and deposits out behind the heater elementl in a solid mass 42. Once the heater element has completed its passage as illustrated in FIGURE 2c, the first section 41, formerly filled with arsenic, is now completely void of arsenic and the second section 43 is now filled with a resolidiiied mass 42.

When it is desired to repeat this process, sections 41 and 42 are interchanged and reassembled in the manner illustrated in FIGURE 2a. The sections are then evacuated and a second pass of the vapor zone is made over the solid charge of arsenic. During each pass of'the vapor zone through the solid charge 41, impurities having a vapor pressure greater than that of the material being refined will be moved in the direction of zone travel and will ultimately be deposited at the end of the charge nearest the nose cap 45. Those impurities having a vapor pressure less than that of the material being refined will deposit behind the zone and will ultimately be deposited at the end of the charge nearest the plug 49.

The methods of this invention although described specifically with respect to arsenic, are equally adapted to the purification of other volatile materials. Such materials include iodine, magnesium, calcium, and zinc, all of which pass directly from a solid state to a vapor state during vapor zone refining. Other materials may also be purified such as cadmium, phosphorus, selenium and sulfur. It is likely that these latter materials will first melt and then vaporize during refinement. In refining such a material, the vapor zone 31 will have on either interface a small liquid zone.

What is claimed is:

1. A method of refining volatile material comprising the steps of providing a charge of said material in an elongated vessel said charge having a length less than that of the vessel whereby to have at one end of said vessel a portion thereof unoccupied by said charge, evacuating said vessel, raising the temperature of the end of said charge adjacent the unoccupied portion of said vessel to at least the temperature of vaporization of said material to produce a vapor zone of said material, and raising the temperature of a progressively longitudinallychanging portion of said charge to at least said temperature of vaporization to cause said vapor zone to travel through the length of said charge in a direction away from said unoccupied portion of said vessel, said vapor zone having a length substantially equal to said unoccupied portion of said vessel.

2. A method of refining volatile material in an elongated tubular vessel closed at one end, said method comprising the steps of inserting a charge of said material in said vessel in contact with the closed end thereof, providing closure means for the other end of said vessel, adjusting said closure means with respect to said charge so as to provide an empty space within said vessel between said closure means and said charge, evacuating said vessel, raising the temperature of the end of said charge adjacent said empty space to a temperature at least equal to the temperature of vaporization of said material to produce a vapor zone of said material, and causing said vapor zone to travel through the length of said charge.

3. A method of refining a volatile material in an elongated tubular vessel comprising the steps of inserting a charge of said material in the solid state thereof into said vessel, providing a first closure means for one end of said vessel substantially in contact with said charge, providing a second closure means for the opposite end of said vessel and positioning said second closure means with respect to said charge so as to provide an empty space within said vessel between said second closure means and said charge, evacuating said vessel, raising the temperature of the end of said charge adjacent said empty space to at least the temperature of vaporization of said material to produce a vapor zone of said mate rial, and causing said vapor zone to travel through the length of said charge.

4. A method of refining a volatile material in an elongated tubular vessel comprising inserting a charge of said material into said vessel, providing a first closure means in one end of said vessel in contact with said charge, providing a second closure means in the other end of said vessel and positioning said second closure means with respect to said charge so as to provide an empty space between said second closure means and said charge, and repeating in sequence the following steps: raising the temperature of the end of said charge adjacent said empty space to at least the temperature of vaporization of said material to produce a vapor zone of said material substantially equal in length to said empty space, causing said vapor zone to travel through the length of said charge whereby successive portions are vaporized and solidified to provide 'a resolidied charge in contact with said second closure means leaving a space between said charge and said first closure means, moving said first closure means in said vessel until it again contacts said charge, and moving said second closure means away from said charge substantially one zone length.

5. A method of refining a volatile material comprising the steps of substantially filling a first section of an elongated tubular vessel with a charge of said material, attaching a second section substantially equal in length to said first section to one end thereof, evacuating both said sections, raising the temperature of a progressively longi tudinally-changing portion of said charge to at least the temperature of vaporization of said material to produce a vapor zone of said material substantially equal in length to one of said sections and causing said vapor zone to travel through the length of said charge in a direction away from said second section.

6. A method of refining a volatile material in an elongated vessel comprising a plurality of substantially iden tical tubular sections each section being adapted to mate end to end with another section in vacuum tight relationship, said method comprising substantially filling one of said sections with a charge of said material, mating a second section with said one section, and repeating in sequence the following steps: evacuating both said sections, raising the temperature of a progressively longitudinally-changing portion of said charge to at least the temperature of Vaporization of said material to produce a vapor zone substantially equal in length to one of said sections, causing said vapor zone to travel through the length of said charge in a direction away `from said second section to cause said charge to vaporize and resolidify in said second section, removing said first section, and mating said second section with another section which then functions as a new second section.

References Cited in the file of this patent UNITED STATES PATENTS Emeis May 2l, 1957 Sequin et al. Dec. 2, 1958 OTHER REFERENCES 

1. A METHOD OF REFINING VOLATILE MATERIAL COMPRISING THE STEPS OF PROVIDING A CHARGE OF SAID MATERIAL IN AN ELONGATED VESSEL SAID CHARGE HAVING A LENGTH LESS THAN THAT OF THE VESSEL WHEREBY TO HAVE AT ONE END OF SAID VESSEL A PORTION THEREOF UNOCCUPIED BY SAID CHARGE, EVACUATING SAID VESSEL, RAISING THE TEMPERTURE OF THE END OF SAID CHARGE ADJACENT THE UNOCCUPIED PORTION OF SAID VESSEL TO AT LEAST THE TEMPERATURE OF VAPORIZATION OF SAID MATERIAL TO PRODUCE A VAPOR ZONE OF SAID MATERIAL, AND RAISING THE TEMPERATURE OF A PROGESSIVELY LINGITUDINALLYCHANGING PORTION OF SAID CHARGE TO AT LEAST SAID TEMPERATURE OF VAPORIZATIO TO CAUSE SAID VAPOR ZONE TO TRAVEL THROUGH THE LENGTH OF SAID CHARGE IN A DIRECTION AWAY FROM SAID UNOCCUPIED PORTION OF SAID VESSEL, SAID VAPOR ZONE HAVING A LENGTH SUBSTANTIALLY EQUAL TO SAID UNOCCUPIED PORTION OF SAID VESSEL. 